Electric cables, in particular for heavy-duty applications and/or for mobile installations, such as mobile harbour cranes, ship-to-shore container cranes, ship un-loaders, spreaders, and mining and tunnelling equipment, are specifically designed to withstand harsh environment conditions and high mechanical stresses, such as bending forces and torques. As a further example of cables for heavy-duty applications, down well pump cables for supplying current to submersible electricity pump systems in deep wells are usually installed in physically restricted areas and in hostile environments, often being in contact with corrosive well fluids. Typically, the above cables are designed to be robust and flexible. Within the present description, we will in general refer to heavy-duty cables, when referring to cables for heavy-duty applications and in particular, but not exclusively, for mobile installations.
An example of heavy-duty electric cable is provided in DE 3934718, which describes an armoured trailing cable for shearer loaders in mines.
WO 01/78086 discloses an electric cable in particular for use in a pick-up system such as a crane or shelving system. The cable comprises a core, which includes first conductors, completely surrounded by and embedded within a first stress-bearing matrix. At least one further layer is disposed about the first stress-bearing matrix and has at least one further conductor in the further layer which is completely surrounded by and embedded within a second stress-bearing matrix. The stress-bearing matrices in the cable are said to allow the distribution of stress throughout the cable and thus to substantially reduce the corkscrew effect.
Bending/compressive loads and twisting in a mobile cable may result from forced guidance of the cable during the winding and unwinding phases around reels or from collection of the cable within baskets (e.g., for spreader cables). In addition, fault or dysfunction of the powered apparatus can bring to the misplacement of the cable that can, for example, drop from the guiding means thereof or be squeezed by apparatus portions thus causing an undue bending of the cable.
Excessive bending of the cable can cause the compressive loads to be transferred to the electrical conductors with consequent damage of the latter. Excessive and/or prolonged compressive loads may result in a deformation of the cable, which would shorten the life of the cable.
U.S. Pat. No. 5,767,956 describes the use of backscattering Brillouin light to provide a monitoring device that is capable of observing, in real time, whether an optical fibre is normal or on the verge of fracture. The device uses optical time domain reflectometry (OTDR) to monitor a stimulated Brillouin scattering light by utilizing one of optical fibre cores in an optical cable. No hint is provided about the use in an electric cable.
WO 08/073,033 describes a system for monitoring the bending and strain of a power cable connected to a moving offshore platform by measuring the strain in optical fibres attached to or incorporated into the power cable. A bend in the power cable will give rise to a strain in the optical fibre and this strain will change the optical properties of the fibre. The change in optical properties can be measured by means of optical time domain reflectometer (OTDR) or optical frequency domain reflectometer (OFDR).
This document states that there exists a risk that the optical fibres embedded or attached to the cable might be damaged and thus it is suggested to equip the cable with redundant fibres. Furthermore, there is no mention of the problem of bending strain transfer between the fibres and the cable to be measured. As possible location for the fibre, the interstices between armouring wires are mentioned.
WO 07/107,693 discloses a fibre optic cable including a strain transfer member, a central optical fibre disposed through the strain transfer member, and a tight jacket mechanically coupling the central optical fibre and the strain transfer member. Strain experienced by the strain transfer member is transferred to the central optical fibre via the tight jacket.
The document does not face the problem of an electric cable with a fibre optic sensor.
The Applicant has been faced with the problem of how to realise an electric cable, in particular suitable for heavy-duty applications and more particularly for mobile installations, which would allow controlling, preferably real-time monitoring, and localizing the bending strain to which the cable is subjected during operation while ensuring long-term reliability of the measurements.